Reclaiming spent doctor solutions



Nov. 27, 1934.

Filed Dec. 24, 1925 FLOW SHEET 2 Sheets-Sheet 2 @aso/me Wafer Was/1 Wafer Wash 00C for Was/7 IN VEN TOR.

Patented Nov. 27, 1934 1,982,577` RECLAIMING SPENT DOGTORSOLUTIONS Robert A.- Dunham;

Long B"e'ach, Calif.; assignor to fUnion Oil Company of` fCalifornim--Los `Angeles, Calif.; a corporation. of: California..` f

Application December 24,

8 i' Claimst.

This"invention-relatestoa process and appara.- tusl for treatingsulfurecontainingfoils With sodium 'c plumbite .or analogous. lead4 coinpoundsxfor` the purpose of removingesulfurof: thez-type known. as sourssulfurg` andfzincludes lmethod andiapparae tusnfor:recovering.theispentzleadicompounds*which havefbeenfpassed.through the process:l Itis parav ticularly f. directed to .the treatment fof gasoline-y like 'compounds .orzpetroleurn fractions onanalo-f. gous fractions vwhichxcontainisulfur. compounds whichrare `reactedonrby, sodium plumbite.- Com-` pounds whichiaretso :reacted .on ane,Lloroadly'speakf-ry ing, hydrogen sulfide and mercaptans that reactx with sodium plumbite. to produce s-leadtsulde and other' `organic` sulfur bodies, resulting.: ini, com'-v poundswhicharenotfurther actedeupo'nuby addi-l tional"I plumbite. Thefoil` containingzbodieswhich reactuwth sodium plumbite is. termed; souriy and".l air-oil whichidoes notl react with plumbite isltermedr: sweet.. i

This plumbite .solutionf whichfisv obtained bythe dissolution of leadoxide (i. e.'-litharge):,.insodiumz hydroxide fis i known 'asv-doctor solution?.

Thefusuafl procedureviswto. mix', for."instance;; gasolinewiththe'doctor solution and, in-,a common;` method, add 'enough sulfur toprecipitatezthe leads which hasio'rrned oil-soluble organic fcompounds.;` offle'ad.y The .precpit'ateis composedrlargely off; lead sulfide;- but i contains also some organic com poundsoi?4v lead.v4 It' has fbeen' proposedlto recover.. leadf from-this vleada-sulrlde `precipitate in: .usable forma One of. thesepropo'sa'ls consists in:` blowing.. air through the lhotgleadr 'sulde precipitate in thet e presenceVV of. sodium A hydroxidev` solution.v 'I'he probable-reactions swhichi :occur are` as `follows :i:

increaseof pressure 'at any temperature: Thus;

. foriexample; in` one vcase theemaximum.:v reaction:

seems to hold irrespective-ot 1928,.Serial v.Noi 328,259

rate was fincreased=three hundreditimes by raising u the pressure fromiatmosphericto -pounds-.-

I havermade thefdiscovery, additionally, that Whereassthe'f rate'oftreaction `atfany pressure-'is increased with increasing temperatures, alimit is soonfreachedf'beyond which the increase'v in .tem- 1 peraturevresults not inH an increase of: reaction.. rate, but 'actually-in: ay rapid: diminution of-reac-, tionfrate such that forzv-onlyfa small-.further in-f crease rof: temperature,y the `reaction rate'will, drop, practicallyito'zero: I haveffoundas the,l boiling point of-l the solution at the pressure is aprproached, `1 Athat f the reaction rate drops off, and.;y thatzwhenithe boiling" pointisfreached the re-. action'y rate t' drops-"to zero; 1- This .phenomenon z fpressure. Forexarnple,thevl doctor-"solution whichv is normally usedztoni'fawgasoline from'fliquid phase-cracking` consistsf'of `about v3 grams of lead oxide 4per .100 cubic'centimeters of solution. The-lead sulde:` precipitate containinglead` sulfide and sodiumv hydroxide-"is'blown with air at 1009 under at--fv mosphericf-pressure, resultinginwan increase of` lead oxide nfsolution of: 0.01 grams per vl100 Acubiccent-imeters per 211011.11.l Ani increase f of. tempera.- ture'toolo F: tripled this reaction rate fproduc.- ing/fsa solution/rate of 0.03-grams `:ofA lead oxide` per r100acubic centimeters-peryhour, but. an in,- crease of temperature from .150? to 180 pro--` duced zno Afurtherrncreasein the reaction rate;L Thereaction .rate was again 0.03 gramsfof lead oxide dissolved per 100 cubicoentimeters of-so1uy tion `per hour. It is to be noted that the'fboiling point 'ofk this vdoctor 'solution at atmospheric ypressure:` iszabout 215- F., If 'this 'Yoxdationr is -car- 90 ried out at 100 pounds absolute andat'150 thenv reactioni'rateI is increased" to. about 2.5 grams per 100 cubicfcentimeters per hourfor about 80itimes as .rapidaasw thefreaction at atmospheric pressure at` .their-same i, temperature.` By raising-,- the tem. perature ,to 200 at .100 pounds'-pressure; the 're-fV action rate isrincreased-` to aboutfgrams eperv 100 cubic centimeters: per hour. On raising thetemperature to" 27 5: the reaction rate -is increased.

tdabOutr'Qgramsper 100 cubic-centimeters per '100 hour; but. on' further raising they temperaturefto 3004 F.- the 'reactionv` ratefalls ytol about 6.5 grams per 100cubiccentimeters per;v hour andzif the tem-l perature is raised to` 325 thereaction falls .to Zero'.` It is to be notedA that theboiling pointA ofthe doctor solutionat-100 poundsis330 F.

While I'dof-notfwish to limit myself vto thefollowingzexplanation of `this peculiar. action, I =be lieve: that` this theory will explain what voccurs. It is well known that chemicalreactions are-:ac-`l `cussed forpurposes of convenience celerated by increase in temperature. Furthermore, this reaction is one between lead sulfide and oxygen and, therefore, will be increased by increase in concentration of oxygen in the solution. There are therefore, two ways in which this reaction will be accelerated; by increasing the temperature, and by increasing the pressure of the reacting gas. The increase in pressure, therefore, has a two-fold effect. It permits the maintenance of elevated temperature in the reaction mixture and also permits increase in partial pressure, that is, in concentration of the reacting gas. However, as the temperature of the material is increased under a giveny total pressure, the vapor pressure of water from the solution being reacted on will increase. This increase in vaporization will decrease the concentration of the reacting gas by decreasing its partial pressure. Therefore, the increase in temperature acts to diminish the reaction rate by increasing the partial pressure of the water vapor and decreasing the partial pressure of the reacting gas. It is well known thatfthe. vapor pressure curves are of logarithmic" character, that is, that the rate of increase of vapor pressure with rise of temperature is slow at low temperature and becomes very steep, that is, rapidly increases, at more elevated temperatures. It is apparent,therefore, that at the lower temperatures increases in temperature do not result in any large increases in vaporization but as the temperature increases this vaporization increases at increasing rates. It is believed that this is iar change in reaction rate the temperature increases.

visualized in the following manner. As the temperature increases, the reactivity,

increases, vaporization also increases, thuslreducing the partial pressure of the reacting.: gas but the increase in temperature has caused a greater increase in reaction rate than the retardation in rate caused by the diminution of concentration of gas. A point is reached as the temperature is increased, when the increase in reactivity, due to increased temperature is more than coinpensated'for by the decrease in rate caused by the diminution in partial pressure of the reacting gas, and therefore, further increase in temperature will cause diminution in reaction rate. This is actually whatv occurs in practice. It is apparent also, that as we approach the boiling point of the solution at the pressure employed, the concentration of reacting gas will drop to zero and that at the boiling point the reaction rate will be zero.

The conclusion which is to be'drawn from the above explanation is that the reaction is best operated at as high a pressure as possible,

and that the temperature shall be as high as possible, but less than the boiling point and in fact, shall be maintained that temperature at which the reaction rate is at the maximum for the pressure`employed.` In the that temperature will be called the optimum temperature and may be deiin'ed as that temperature at any givenpressure .at which the reaction rate is at the` maximum.- The reaction rate as herein disis the rate of formation of lead oxide in solution as plumbite, per unit of time, as for instance in the illustrative examples it is the rate of increase of lead oxide in grams per hundred cubic centimeters or solution per ho'ur.

substances in the the explanation of the peculn at anypressure as-V The change may beiA i. e., the re. action ratefor a` system of given composition' preferably not greater thank The above reaction converts the lead sulde into sodium plumbite which may be used directly for further treatment of material such as sour gasoline. The separated lead sulfide sludge may be further regenerated and used over again. However, I have found that unless precautions are taken to remove emulsifled gasoline from the doctor solution before it is regenerated, the regenerated doctor solution will discolor the treated gasoline. Gasoline, especially gasoline produced by a cracking process, on contact with doctor solution at elevated temperature is very rapidly discolored. Thus if the lead sulfide-alkali mixture contained emulsified gasoline the regenerated doctor solution when used over again would introduce these highly colored bodies into the gasoline which is beingtreated. Additionally the oxidation of emulsifed bodies would produce organic acids which would increase the danger of emulsiiication when doctor solution is used over again. It is, therefore, highly desirable to separate the emulsifled oil from the doctor solution before regeneration. Anadditional precaution is advisable to prevent the accumulation of regenerated doctor solution which would injure the oil subsequently treated. lSome gasolines contain acidic compounds, either organic or in-organic which are preferably removed by a precaustic .treatment before doctor treatment. This is especially true of gasoline treated with sulfuric acid and adsorbent clay and which would not otherwise receive an alkali treatment.

An additional modification which increases the rapidity 'of regeneration is the introduction of excess lead sulfide. Normal doctor solution, as has been indicated above, contains an amount of .lead oxide equal to about 3 grams per 100 cubic centimeters and the spent doctor solution will therefore contain very little more than 3 grams of PbS perv 100 cubic centimeters. The purpose of reaction is to convert this lead sulfide back to lead oxide. Instead of oxidizing the spent doctor solution as such, an amount of lead suliide is added to the spent doctor to increase the lead sulfide content beyond that existing in the spent doctor. This increase in PbS will increase the solution rate. The recovery may be stopped when the solution contains 3 grams of PbO per 100 cubic centimeters and removed from the excess PbS, or the operation may be continued until the precipitate is nearly all dissolved, or until the solution contains as much as 6 to 9 grams of PbO per 100 cubic centimeters; this solution subsequently being diluted to proper strength by the addition of caustic solution. This invention will be better understood by reference to the drawings which accompany this case.

Figure 1 is a schematic illustration of an apparatus for carrying out this process.

Figure 2 is a-flow sheet showing the course of the material.

Referring to Figure l, gasoline which has received its acid and clay treatment to produce a gasoline of proper color is passed through 1, by means of pump 2 into an injector-type mixer 3, where it is co-mingled with separated alkali passing from separator 37 as will be further explained. This alkali is pumped by means of pump 41 -into the mixer 3. This mixer operates to regulate the flow of alkali and gasoline at the proper proportions and to insure intimate mixture. The co-mingled alkali and gasoline is passed to separator 6 from which separated alkali is withdrawn through draw-off line 8. The alkali treated gasoline I is withdrawn 4through 'draw-effi is washed-1in 9 "with 'water "enterin'gr The-co-rriingled'-gasoline and waterl pa'ssesrintoa separator 12 fromfwhich water withdrawn through 13 to the sump and gasolineis withdrawn by meansbf'pump 14 and pumped through`- line 15 into'A mixer 16 fwherein it visf 'cemingledwith doctor solution passing from-'thesecond' doctor treatment as -willv be hereinafter described.- Thisdoctor solution is-pumpedby pump 25into mixer' 16--and isy (zo-mingled with;

gasoline from' l5; The -comingled material is passed to separatorI 18 -f-rom which the spent doctorv solution is `passedvthroughl line 20'fand pump 35 zto the regeneration step which will l be hereinafter described. The gasoline" -passing from'l8 through line A'i9 is pumpedby pump 20 into mixer-21 hereritis'co-mingled -with doctorl solution pumped by pump 81" through line 22 intof mixer2l. The'co-mingled material isv passed into `separator A24 and, as hereinbefore described,`

the doctor solution is pumped by pump 25into mixer 16 and the twice ltreated gasoline is passed through 26 by pump'27 to 28 where-it receives la water-wash by means ofwater introduced through 29. into separator 31 and theV finished gasoline is drawn-'through' 32 while water is Withdrawn through 33 and pumped bypump 34` linto 9ashereinbeforedescribed.

The spent doctorfsolution', as before stated, is

pumped by pump 35 into heater 36A where it is heated by -steam'and-'Ipassed into separator 37. In37 the venfxulsifed oil stratines and rises in compartment 33" andV is withdrawn through launders 39.V y

It is possible that oil vapors, generated by heating thematerial, may-bel liberated in the first compartment of the decanter and interfere with as'hereinbefore described, is pumped to the precausti'c treatment in 6, the remainder passing to 67 'and-63'. Part ofthe caustic may be diverted via 64:` (a) bypropermanipulation of 10 (a) to dilute th'eregenerated solution as described above. The leadl sulfide with its accompanying alkali solutionis"introducedrinto 43.1 The purposeof this tank lisf-to permit the continuous decanter to loperate in conjunction withfa continuous digester.. Itshould have the capacity offat leastk twice the amount of vmaterial required to charge the di@` -gesteri` It isrto be keptfromone-fourthto threefourths* full in: normal operation,v `providingsuii cient: reserves of material and storage space to care -for' any temporary laclrof coordination be-V tweeny thepreceding and succeeding operations., Thegmaterial in the intermediate storage tank.` Willrhave tov be agitated` suiiciently to ,prevent` segregation, so .as to supplymaterialto the-digester containing itsproper quota of precipitate.

The agitationfmay be either by low pressure air or the vent gases from the digester may be einh ployed.` Line fle isprovided for the introduction of4 air and'45i's for. theintroduction of vent gases. The material is withdrawn from storage tank 43 via line. 47' through pumpe and passed through preheater LligandV introduced into digester 51 via The co-mingled material-L is passed via 30- material "mayffbe circulated' and agitatedA and whereby fthe* digester' mayfbeemptiedaiinto 43 if "desired, Valve vSlaandValVe 611, whenproperly adjusted willpermitthis by-passing. The materialy is pumpe'dinto 5l" when the valve6la is closed, thel other==valves being `properly adjusted.

Steamis passed through heater 52 and ai-r is introduced under hig-h pressurethrough line'53,

valvef54 and 'nozzles 55." 'Valve 56bis closed and' .valve 56a is regulated to maintain the desired pressure in 5l. Some of thevent gas is passed through 56, 45 and 45d to 'maintain agitation in c13.` The residual gas isI passed through''Y-f and is re-compressed by recompressing vblower 58'and reintroduced through 59by lnozzles 60. Thev re action is carried on for the desired length of time.`

When reaction has been completed, valve 54"-is closed, recompression blower 58` is stopped andthe charge is allowed to stand-for a time sufiicient to` precipitate most of the suspended solids. The' clear solution is blown'out by closing 54- and open#v ing valve 56h and closing valve 56a'. The air pressure will blow the niaterial'through 64 into" tanks 67 and 68. Byproviding another digester similar to '51* `in parallel therewith, the process may be continued while 51 is being emptied. The material is introduced into` tanks 67' and `68,"as

hereinbefore stated, depending upon whichr one is available, by opening the proper valve 65 and 66. These tanks arethe storage tanks from' which the materialmay be withdrawnby pump'l via lines 79 or 80.. Additional makeup` tank 73' is provided in which additionalV plumbite may be generated by lead oxide introduced into. the tank.

Water is` introduced via 7l, ',controlledby Valve' 72, caustic solution through line 69, controlled by 70. Makeupy may thenl bepumpejd .by pump 74' into line '75, controlled by ,Valve '77,' and/or through line '76, controlled by Valve 78. The operation of the plant is as follows: Gasoline, for instance cracked gasoline which is given a .sulfuric acid .treatment followed by a: treatment with decolorizing and neutralizing. clayk is treated with doctor sclutionin the process described-above.

Standard doctor solution, for example, may be composed of 3 gramsoflead oxide and 10 grams of sodium hydroxide. per 1,00 cubic centimeters. When the solution is `used in correcty proportions,

all the lead is precipitated chiefiy as lead sulfide.V

All the caustic except for some that is neutralized by acid constituents ofthe gasoline, remains inv the. solution unchanged. Thus the caustic solu'- tion may be regarded like the water, as merely a Vehicleorcarrying. the lead oxide which is the actual reagent;

previous acid clay treatmentis usually about 6% byv volume, of the oil treated.

`As has been previously pointed rout, it is ad-` visable to insure that the spent doctor. solution?v is not contaminatedby color `bodies inorder that.` the reclaimeddoctor solution be as pure as pos. sibleand not contaminate the freshgasoline tofbe treatedthereby. Inorder `to accomplish this ythe gasoline `receives a precaustic wash followed by a water wash before `it is treated by the docto-r solution, In theprocess here describedthe caus-A tic is obtained by settling `of `the spent doctor. When the spent doctoris treated to remove the emulsiied oil this vsettling occurs at '37 and the clear alkali solution is'removedas previously des scribed by line 40 vand pump'411 It is then; intro-` The amount of` 'doctor solution required totreat algasoline which has received a 1 lll duced as previously described by means of the mixer 3 together with gasoline into separator 6. The spent caustic is removed and this caustic treated gasoline is then water Washed. The Water used for this water wash is that which has been previously used to wash the gasoline passing from the doctor treatment. 'I'hus the fresh Water is introduced into 28 to Wash the doctor treated gasoline and the water separated in separator 31 to pass into Washer 9 Where it meets the caustic treated gasoline and is nally discharged through 13. The gasoline is now ready for its doctor treatment. A two stage counter-current treatment is employed. The doctor solution is composed in substantial part of recovered doctor solution accumulated in tanks 67 and 68. Due to the fact that some lead is lost and due to the fact that some alkali is used up in the caustic treatment, make up is added from tank 73. Tank 73 is used as a mix tank and the doctor solutionk there mixed is added to 67` and 68 to bring it to standard strength. The doctor solution passes counter-current tothe caustic and water washed gasoline in two stages. Gasoline passes first into stage 1, and then intostage 2, and doctor solution passes counter-current iirst, into stage 2, and then into stage 1. See flow sheet, Figure 2. Stage 2 represented in Fig. 1, by mixer 21 and separator 24. Stage 1, is represented by mixer 16 and separator 18. The water washed gasoline from separator 12 is passed by pump 14 intoY mixer 16 where it meets the once used doctor solution from separator 24. The doctor solution is removed by line 20 and pump 35 for regeneration. The once treated gasoline then passes into stage 2 by line 19, pump 20 and mixer 21 where it meets the doctor solution from tanks 67 and 68, passingvia pump 81 and line 22, into mixer 21', than passing to separator 24. The gasoline from separator 24 is removed by line 26 and pump 27 and introduced into 28 where it receives its water wash. The mixture of Water and gasoline passes to line 30 into separator 31, finished gasoline being removed through 32 and the water being removed by line 33 and pump 34 as previously described, to be usedto wash the 'gasoline which has received its precaustic wash.

The regeneration of the spent doctor will now be described.

The rst step in this reclamation process is the removal of emulsiiied gasoline in the spent doctor. The spent doctor is pumped by pump 35 through heater ,36 which may be heated by exhaust steam and is preferably heated to 160 F. If the treatment is continuous the hot doctor solution from the digester 51 could be used as a source of heat in heat exchanger 36. The thus heated spent doctor solution then passes into decanter 37. The emulsied oil which is stratied is overflowed through launder 39 as previously described. Some segregation of lead precipitate and caustic occurs. The caustic is removed through line 40 as previously described, in necessary amounts for the precaustic treatment. About 10% of the caustic is thus removed and also such caustic solution as is not required in the digester may be by-passed to 67 and 68.

The remainder of the caustic and the spent lead areunderflowed by line 42 into intermediate tank 43. This tank is used when discontinuous digestion is employed. If the digestion is of a continuous nature this tank would be by-passed `and the digester fed directly from the decanter.

The purpose or this intermediate stage tank 43 is to permit the discontinuous decanter tc operate iny conjunction with the discontinuous digester. It should have a capacity of at least twice the amount of material required to charge the digester. It is to be kept from one-quarter to three-fourths full in normal operation, providing suicient reserves of material and storage space to take care of any temporary lack of coordination between the preceding and succeeding operations. The material in the intermediate stage tank will have to be agitated sufficiently to prevent segregation so as to supply material to the digester containing its proper quota of precipitates. To accomplish this, air is blown through the tank. Vent gases from the digester may also be employed as in Fig. 1, vent gases being introduced through 45. This agitation will cause some evaporation and the temperature of the material in the vintermediate tank 43 will adjust itself to about 140 F.

The material is withdrawn through line 47 and pump 48 and is heated in heater 49 to about200 F. The digestion in digester 51 will be carried on as previously described in the presence of an excess lof spent lead. That is, at the start of the digestion there will be present an amount of precipitate equivalent to from 6 to 12 grams of PbO per 100 cubic centimeters of solution, and at the end of the digestion about 3 grams per 100 cubic centimeters; from 3 to 9 grams per 100 cubic centimeters having passed into solution. The remaining precipitate will be left to supply the excess for the following charges. 'I'his arrangement maintains the reaction rate at a high point throughout the Whole of the digestion period. Without this additional amount of lead, the reaction rate would drop off as the lead is dissolved.

During the digestion fresh air will be supplied through line 53 into spider 55 under an elevated pressure ofeabout 95 or 100 pounds absolute. The gases are vented through valve 56a to maintain this pressure and steam is passed through heater 52 to maintain the-elevated temperature of about 200 F. The air which is blown into the digester 51 acts both to oxidize the lead sulde and also to agitate the mixture to keep the precipitate in suspension and thoroughly mixed. When more air is required for agitation than is required to supply oxygen for oxidation, it has been found economical to recycle vent gases via line 57 and re-compression blower 58 and introduce the same through line 59 into the digester 51. Under these circumstances it is only necessary to supply through 53, the air necessary for oxidation, the agitation being supplied by the re-introduced gases through 59. This digestion continues until the desired solution is obtained.

At the end of the treatment the air is shut off as previously described and the excess lead suliide allowed to settle. This settling occurs Within an hour. The clear liquor is discharged through the side opening and, valve 64a being opened, air pressure, introduced by means of 53a with the proper adjustment of valves of 56a and 56h as previously described, is employed to discharge the material. The excess lead sulfide remains in the digester and is ready for a second treatment. The recovered doctor solution is discharged through line 64 and receives its make up in tanks 67 and 68.

It is to be observed that whereas the maximum obtainable reaction rate at 100 pounds absolute is at275 F., the reaction here carried out Was at 200 F. The reason for this is that it is more economical where exhaust steam is available to operate at a lower temperature.

In general, the conditions chosen for a reaction in commercial operation are those which will give a product of lowest cost consistent with satisfactory quality. Reaction rate enters into a consideration of cost principally in that it affects the size of equipment required for a definite daily production. The size of equipment in turn affects the operating cost because of the necessity of making an allowance for maintenance, interest, and depreciation as a percentage of the initial cost of the equipment. It should be pointed out, however, that these items are not necessarily the most important factors in the total cost of operation. In the present case, the point of maximum economy will probably be found to be somewhat before the point of maximum rate. The reason for this is that at the point of maximum rate and beyond it, there will be a large amount of evaporation of water from the solution which, in turn, will necessitate supplying large amount of heat to furnish the heat of vaporization. In fact, as the point of maximum rate is passed, the quantity of water vaporized approaches infinity as the rate approaches zero. It is therefore advisable as is here described to operate below the temperature of maximum reaction rate. However other conditions may make advisable the operation at as near that temperature as possible. The man skilled in the art will be able to use proper adjustments.

While I have spoken of using air as the most useful oxidizing agent, other oxidizing gases such as pure oxygen or ozone may be employed.

In like manner whereas the carrying agent has been described as sodium hydroxide other equivalent caustics may be used as well as other equivalent reagents.

The above disclosure is therefore not to be taken as limiting the scope of my invention but is merely illustrative thereof and of the best manner of carrying this out and can be modified within the scope of my invention which I claim to be:-

1. A method for regenerating spent doctor solution containing lead sulfide which comprises heating the lead sulde and contacting the said lead sulfide with an oxidizing gas under superatmospheric pressure and at a temperature below the boiling point of the solution at such superatmospheric pressure.

2. A method for regenerating spent doctor solution containing lead sulfide which comprises heating the lead sulfide and sodium hydroxide solution under superatmospheric pressure and contacting the said lead sulfide with an oxidizing the said lead sulde with an oxidizing gas and maintaining the temperature of the reaction not above `the optimum temperature for aforesaid pressure.

4. A method for regenerating spent doctor solution which comprises treating gasoline with doctor solution, withdrawing the gasoline from the spent doctor solution, adding additional lead sulde to the spent doctor solution, contacting mixture with an oxidizing gas at a superatmospheric pressure at an elevated temperature, but not above the optimum temperature for such pressure, and withdrawing the thus regenerated doctor solution.

5. A method for regenerating spent doctor solution which comprises treating gasoline with doctor solution, withdrawing the gasoline from the spent doctor solution, adding additional lead sulde to the spent doctor solution and contacting the mixture with an oxidizing gas at an elevated temperature and at a superatmospheric pressure and maintaining the temperature at not above the optimum temperature at said pressure and withdrawing regenerated doctor solution.

6. A method for regenerating spent doctor solution containing lead sulphide which comprises heating lead sulphide and sodium hydroxide solution under a pressure from 40 to 100 lbs. per square inch, and contacting the said lead sulphide with an oxdizing gas and maintaining the temperature of the reaction not above the optimiun temperature for said pressure.

'7. A method for regenerating spent doctor solution containing lead sulphide which comprises heating lead sulphide and sodium hydroxide solution under a pressure of 100 lbs. absolute, contacting the said lead sulphide with an oxidizing gas and maintaining the temperature of the reaction between 200 and 300 F.

8. In aimethod according to claim 7 wherein the reaction is carried out at the temperature of about 275 F.

ROBERT A. DUNHAM. 

